In internal-combustion engines, it is commonly used to let the combustion process take place within a cylinder whereby a piston is forced to move relative the cylinder. The relative movement has to experience a low friction in order not to waste the energy released by the combustion process and particularly not to transfer the released energy into heat in the piston and cylinder. Furthermore, the physical relation between the piston and the cylinder has to be such that any leaks of combustion gases are reduced to a minimum.
To this end, the inner surface of the cylinder is carefully treated, in order to reach a final surface roughness typically in the range of Sa=0.15-0.50 μm. Such a surface treatment process is normally performed in a number of steps; boring, rough honing, fine honing, plateau honing and possibly running-in of the cylinder against the mating piston ring. The resulting surface profile often consists of a plateau shape stylus with flat summits and valleys available for containing lubricant, i.e. lubricant reservoirs.
During operation of the piston and cylinder, a lubricant is usually added. The remaining roughness in the cylinder walls can contain small volumes of lubricants, which provides a film between the cylinder and the piston, giving rise to relatively low friction coefficients, i.e. full film lubrication. However, as the sliding speed approaches zero at the turning points of the piston the requirements for full film lubrication is not fulfilled. In this regime, called boundary lubrication, the friction coefficient is determined by the shearing properties of the two solids in contact; piston ring material and cylinder wall material.
The traditional lubricant is based on a petroleum product. When coming into contact with the hot environment in the cylinder, some of the lubricant will also decompose. Since the lubricants often comprise not so very environmentally friendly elements, such decomposing of the lubricants can give rise to hazardous combustion gases. There is therefore a need for reducing such addition of hazardous lubricants for environmental reasons. Maintaining good lubricity between the piston ring and cylinder will though be difficult without such lubricant additives.
Alternative lubricating substances, such as solid lubricants, have also been used. Graphite, MoS2 and WS2 are e.g. known to exhibit low friction properties. In WO95/02023 a cylinder bore wall of an engine is provided with a thermally sprayable powder comprising a core of at least graphite and MoS2 encapsulated in a thin metal shell of a soft metal such as e.g. Ni or Sn. The coating also provides a porosity in which oil lubricants may be retained. In the English translation of the abstract of CN1332270, a method is disclosed in which low friction surfaces are provided by electroplating or chemical plating in plating liquids containing MoS2 or WS2. In GB 847,800, metal sulfide coatings are provided by thermal decomposing of polymers containing e.g. W and S.
Curved surfaces, and in particular inner cylinder walls, present a particular challenge for surface treatment. Surface coatings based on spraying, electroplating, thermal decomposing, PVD, CVD etc. are difficult to provide in a smooth, even and controllable manner over the entire surface. The reason is mainly geometrical, since equipment or substance supplies have to be performed in the typically restricted volume inside the cylinder and also subject to possible shadowing effects. Entirely new manufacturing process steps and manufacturing tools have to be provided, which makes the production costs very high.
Furthermore, the solid lubricant layers provided by prior art methods have different kinds of inherent drawbacks. In cases powders in soft metal shells are utilized, the lubricant properties of the core are partly prohibited by the soft metal. Furthermore, the lubricant substance of the core is provided in an arbitrary crystal direction thereby presenting both low friction surfaces and surfaces with somewhat higher friction. In the case of electroplating or thermal decomposing, the adhesion of the surface layer to the cylinder wall is difficult to control, as well as any crystal growth direction. Furthermore, adapted reaction environments have to be provided.